The bones of the carpus, exclusive of the accessory, form an irregular
quadrangular mass, the width of which is about twice the height or the
antero-posterior diameter. The =anterior= or =dorsal surface= is convex
from side to side, depressed along the line of junction of the two rows,
and prominent below. The =posterior= or =volar surface= is in general
slightly convex, but very irregular. It forms with the accessory the
=carpal groove= (Sulcus carpi), which in the recent state is rendered
smooth by the posterior ligament; it is converted into the =carpal
canal= (Canalis carpi) for the flexor tendons by the transverse carpal
ligament, which stretches across from the accessory bone to the inner
side. The =proximal surface= is widest internally and is elevated in
front, concave behind; it is entirely articular and adapted to the
carpal articular surface of the radius. The =distal surface= is also
articular and is irregularly faceted in adaptation to the surfaces of
the metacarpal bones; each of the lower bones usually articulates with
two metacarpal bones, but sometimes the third does not bear on the inner
metacarpal bone. The =lateral surfaces= are both irregular and rough,
the internal one being the wider. With the exception of the accessory,
ulnar, and second, each bone articulates with two bones of the other
row.
=Development.=—Each ossifies from a single center.